In recent years, there have been rapidly proceeded the research and development for a high-density optical disc system, which is capable of recording and/or reproducing information (hereinafter, “recording and/or reproducing” will be expressed by using following wording “recording/reproducing”) by using a blue-violet laser diode having wavelength of about 400 nm. For example, there is provided an optical disc, what is called a “Blu-ray Disc” (hereinafter it will be called BD), used for recording/reproducing information based on the standard that NA (Numerical Aperture) is 0.85 and wavelength is equal to 405 nm. As for BD, information of 23-27 GB per a layer can be recorded on the optical disc having a diameter of 12 cm, which is the same size as a DVD which is used based on the standard that NA is 0.6 and wavelength of a light source is 650 nm, and whose recording capacity is 4.7 GB. There is also provided an optical disc, what is called a “HD DVD” (hereinafter it will be called HD), used for recording/reproducing information based on the standard that NA (Numerical Aperture) is 0.65 and wavelength of a light source is equal to 405 nm. As for HD, information of 15-20 GB per a layer can be recorded on the optical disc having a diameter of 12 cm. These discs are named “a high density optical disc”.
On the other hand, it is sometimes considered that a product, such as an optical disc player and a recorder (hereinafter it will be called an optical disc player/recorder), which is capable of only recording/reproducing information for a high-density optical disc is worthless. Taking account of a fact that, at present, DVDs and CDs (Compact Disc), onto which various kinds of information have been recorded, are on the market, the value of the product as a high-density optical disc player/recorder is increased by, for example, enabling to appropriately record/reproduce information additionally for DVDs and CDs, which user possess. From these backgrounds, the optical pickup apparatus installed in the high-density optical disc player/recorder is required to be capable of appropriately recording/reproducing information not only for a high-density optical disc but also a DVD and a CD.
As a method for appropriately recording/reproducing information for any disc of a high-density optical disc, DVD and further CD while keeping the compatibility, it is feasible that a method of selectively switching the optical systems corresponding to the recording density of discs: the high-density optical disc, the DVD and further the CD, for which information is recorded/reproduced. However, since a plurality of optical systems is required for the method, it is disadvantageous for the minimization of the product and which increases the cost of the product.
So, it is preferable to commonly share the optical system for the high-density optical disc and the optical system for the DVD and CD as much as possible in an optical pickup apparatus having compatibility in order to simplify the structure, to decrease the cost of the optical pickup apparatus and to decrease the number of optical parts structuring the optical pickup apparatus as much as possible. Further, to commonly share the objective optical system, which is placed so as to be opposing to the optical disc, has advantages from the viewpoints of simplifying the structure and decreasing the cost of the optical pickup apparatus. In addition to this, there is a requirement that an optical detector for receiving a reflected light flux from the optical disc needs to be commonly used.
Meanwhile, Japanese Patent Application Publication Open to Public Inspection (JP-A) No. 2005-141800, has disclosed an optical pickup apparatus having a commonly used objective lens realized by inserting a divergent angle conversion element in the optical pass on which light fluxes used for DVD and CD travels.
The performances necessary for the optical system in an optical pickup apparatus depend on the respective applications in the optical pickup apparatus. For example, chromatic aberration correction is unnecessary to only read an optical disc, particularly a high-density optical disc and a DVD. However, the chromatic aberration correction is necessary to record information onto the high-density optical disc and the DVD. In the optical pickup apparatus disclosed in JP-A 2005-141800, the chromatic aberration caused when the optical pickup apparatus is used with the high-density disc and the DVD is controlled in a low level by providing a diffractive structure on the divergent angle conversion element. In general, since the dispersion characteristics of the material are different in respective wavelength ranges, the amount of the chromatic aberration of the high density disc and the amount of the chromatic aberration of the DVD are different. In the optical pickup apparatus disclosed in JP-A 2005-141800, there is provided different coupling lenses: one is the coupling lens through which a light flux for the high-density optical disc passes and the other is the coupling lens through which a light flux for the DVD/CD passes. Using the different coupling lenses to good advantage, each of the coupling lenses corrects the chromatic aberration with the corresponding amount. When a common coupling lens is used to lower the cost, there is a problem, which is how to compatibly realize the correction of the chromatic aberrations for the high-density disc and the DVD/CD is realized.